The present invention relates generally to positron emission tomography (PET) and magnetic resonance (MR) imaging, and more specifically, to a combined PET-MR system and method for excluding PET data detected during MR transmissions to improve overall data quality of a PET scan.
PET imaging involves the creation of tomographic images of positron emitting radionuclides in a subject of interest. A radionuclide-labeled agent is administered to a subject positioned within a detector ring. As the radionuclides decay, positively charged photons known as “positrons” are emitted therefrom. As these positrons travel through the tissues of the subject, they lose kinetic energy and ultimately collide with an electron, resulting in mutual annihilation. The positron annihilation results in a pair of oppositely-directed gamma rays being emitted at approximately 511 keV.
It is these gamma rays that are detected by the scintillators of the detector ring. When struck by a gamma ray, each scintillator illuminates, activating a photovoltaic component, such as a photodiode. The signals from the photovoltaics are processed as incidences of gamma rays. When two gamma rays strike oppositely positioned scintillators at approximately the same time, a coincidence is registered. Data sorting units process the coincidences to determine which are true coincidence events and sort out data representing deadtimes and single gamma ray detections. The coincidence events are binned and integrated to form frames of PET data which may be reconstructed into images depicting the distribution of the radionuclide-labeled agent and/or metabolites thereof in the subject.
MR imaging involves the use of magnetic fields and excitation pulses to detect the free induction decay of nuclei having net spins. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
These magnetic fields and RF pulses of MR imaging can affect the function of PET components, and consequently, the reliability of acquired PET data. In hybrid systems which combine these two modalities, the magnetic fields and RF pulses of the MR components can affect the PET detector ring components to varying degrees. For example, photomultiplier tubes and other photovoltaics do not function very well in magnetic fields. Similarly, RF pulses can cause increased noise in the PET detection signals. Thus, PET data acquired at the time of an RF or gradient pulse might be considered suspect and, in some instances, may be unusable for PET reconstruction.
Some attempts at compensating for the effects of MR transmissions on PET data have included the use of RF shielding on and about the PET components. However, these techniques may not always be completely effective at eliminating the effects of MR transmissions on PET components and detected data quality, especially in higher tesla MR imaging. In addition, the inclusion of RF shielding increases the cost and complexity of hybrid MR-PET systems. Other attempts at compensating for MR interference with PET acquisition have included the use of light pipes to convey scintillator illuminations to remotely located and shielded photovoltaics. Such methods also increase the cost and complexity, as well as the size, of MR-PET scanners.
It would therefore be desirable to have a system and method capable of efficiently and effectively compensating for the effects of MR gradient and RF pulse transmissions on PET equipment without the need for complex RF shielding or other additional physical compensation components. It would be further desirable if the system could adapt to perform such compensation in various configurations of existing hybrid PET-MR scanning systems.